86 



THE ECOLOGICAL RELATIONS OF ROOTS. 



Both large and small rootlets may arise from the rhizomes. The main 

 laterals are well supplied with rootlets and the soil is quite filled with them 

 to a depth of 6 to 36 inches. As compared with Aletes and Apocynum, the 

 rootlets are coarse and rather poorly branched. They vary from tan to nearly 

 white in color. The tap, as in many gravel-slide plants, is rather fleshy. 



Fig. 35. — Pachylophus caspiiosus. 



Thlaspi alpestre. — This penny-cress is abundant but rather inconspicuous, 

 being only 5 to 8 cm. high. Its tap-root is usually not over 2 mm. in diameter, 

 and penetrates the dry surface gravel to a depth of 1.5 to 2 inches before it 

 gives off branches. Beginning at this depth, tiny laterals, ranging from almost 

 microscopic in size to 1 mm. in diameter, develop in abundance (plate 24, d). 

 The larger laterals are relatively few, usually not more than 2 to 5, but the 

 smaller ones, many about the size of a fine silk thread or smaller, occur in 

 great abundance. Often they run parallel with the surface throughout the soil, 

 branching repeatedly in such a manner as to form a delicate mass of rootlets, 

 the ultimate termini being minute. The lateral extent is usually not greater 

 than 4 to 7 inches and the depth of the plant seldom exceeds 15 inches. To 

 really appreciate the dehcacy of the profusely branched fragile root system, 

 it is necessary for one to carefully pick away the coarse rock particles. 



